Cold seal release film with improved scuff resistance

ABSTRACT

A cold seal release film in the form of a monolayer film or an outer layer of a multilayer film. The cold seal release film includes a nucleating agent and 10%-100% by weight of the polymer composition of a polymer selected from the group consisting of poly-1-butene homopolymers, poly-1-butene/ethylene random copolymers having over 50% by weight poly-1-butene therein and a blend of the poly-1-butene homopolymers and poly-1-butene/ethylene random copolymers, and 90%-0% by weight of the polymer composition of a polymer selected from the group consisting of ethylene random copolymers of polypropylene, butene random copolymers of polypropylene and ethylene-butene random terpolymers of polypropylene.

BACKGROUND OF THE INVENTION

[0001] This invention relates generally to cold seal release films, andmore particularly to cold seal release films having improved scuffresistance.

[0002] Cold seal release films are most commonly used as the outside webor film of a multi-web packaging structure. In common practice, therelease film is a clear web that is reverse printed and adhesivelaminated to an inner web such as an opaque film (in a candy bar wrap)or a metallized film (in a breakfast bar wrap). After the two films arelaminated a layer of cold seal adhesive is applied to the inner web asthe web is wound into a roll. The cold seal release film serves thefollowing functions: 1) it provides a transparent web which can bereverse printed so that the decoration is “buried” beneath the clear webto prevent the ink from being worn away; 2) it provides the desired slipproperties so that the laminate will run effectively on packagingmachines and 3) it provides a surface with low adhesion to the cold sealadhesive so that the laminate roll can be unwound during a packagingoperation, with the adhesive remaining on the desired surface of thelamination.

[0003] The most successful prior art cold seal release film in themarket place today employs a polypropylene homopolymer as the cold sealrelease surface. This prior art film provides good printingcharacteristics, low coefficient of friction (COF) and a reasonably lowadhesive release force from the most common cold seal adhesives; namely,acrylic/rubber latex adhesives. Moreover, prior art cold release filmsemploying a polypropylene homopolymer also have reasonably good scuffresistence. However, the prior art films employing polypropylenehomopolymer have a major deficiency; namely, they do not provide thedesired low adhesive release force from synthetic adhesives, e.g.,ethylene vinyl acetate based adhesives.

[0004] Other polymer films containing poly-1-butene homopolymer,poly-1-butene/ethylene copolymers (collectively referred to aspolybutenes) and poly-1 butene copolymers blended with ethylene and/orbutene random polypropylene copolymers yield low adhesive release forcefrom both synthetic adhesives and acrylic/rubber latex adhesives.However, these prior art films do not have desired scuff resistenceproperties. Representative patents disclosing these latter polymercompositions for cold release films are U.S. Pat. Nos. 5,981,047;5,798,174; 5,489,473 and 5,792,549, which are all incorporated herein byreference.

[0005] As noted above, an important characteristic for cold seal releasefilms is good scuff resistence. This is an important property becausethe film serves as the outside web or layer in a monolayer or multilayercold seal packaging application. Therefore, scuffing, which can occureither during the printing/laminating process or the packaging process,causes the finished product to have a dull or shop-worn appearance,which is undesirable.

[0006] The following additional patents also disclose films employingbutene polymers:

[0007] U.S. Pat. No. 4,554,321 to Hwo et al. discloses heat sealablefilms having improved seal impact strength comprising a stereoregular,elastomeric polymer of butene-1 or blends thereof with conventionalbutene-1-ethylene copolymers, optionally with small amounts ofpolypropylene and/or crystallization nucleating agents. The use of suchstereoregular, elastomeric polymers is not desirable for use in coldseal release films of this invention.

[0008] U.S. Pat. No. 5,998,039 to Tanizaki, et al. discloses apolypropylene composite film comprising (1) a crystalline polypropylenelayer and (2) a layer formed from a blend of a polypropylene homopolymer(5-95%) and a propylene 1-butene random copolymer (95-5%), wherein thepropylene component is greater than 50%, by weight, of the copolymercomposition.

[0009] WO 00/32395 A1 to Chang et al. discloses a polypropylene filmcontaining a non-migratory slip and release additive package of asynthetic or mineral wax, an aluminosilicate additive, and optionallysilicone oil and a cross-linked silicone polymer resin to improve coldseal adhesive release properties. Although this patent discloses addingnucleating agents to a polypropylene film for improving the strength ofthe film, there is no disclosure or suggestion that nucleating agentsshould or could be added to a polymer composition including a poly-1butene homopolymer or copolymer composition.

[0010] Nucleating agents are known to increase the stiffness ofprocessed materials. Nucleating agents also are known to improve theoptical and barrier properties of the resulting materials. Sodiumbenzoate is a desirable nucleating agent because, for among otherreasons, it is readily available and economically practical to use.

[0011] However, the use of nucleating agents such as sodium benzoate inthe production of biaxially oriented polypropylene films has beenconsidered to be undesirable because, in the amounts typically employed,it creates processing difficulties. U.S. Pat. No. 5,837,177 to DiNardoet al. discloses a method of making a polypropylene product for use inmaking biaxially oriented films. The method includes the addition of animproved nucleating system that allows for even distribution of anucleating agent in a polypropylene material. The improved nucleatingsystem comprises a nucleating agent such as sodium benzoate and a liquidmiscible with the nucleating agent. The solution is combined with thepolypropylene material, and the miscible liquid is removed duringprocessing.

[0012] There is a need in the packaging field for an improved cold sealrelease film exhibiting both a low adhesion to cold seal adhesives,including synthetic adhesives, and improved scuff resistance. It is tosuch films that the present invention relates.

[0013] All references cited herein are incorporated herein by referencein their entireties.

SUMMARY OF THE INVENTION

[0014] A cold seal release film constituting either a monolayer film oran outer layer of a multilayer film includes a nucleating agent, such assodium benzoate, provided in a polymer composition comprising 10%-100%by weight of the polymer composition of a polymer selected from thegroup consisting of poly-1-butene homopolymers, ethylene randomcopolymers of poly-1-butene including over 50% by weight poly-1-buteneand blends of the poly-1-butene homopolymers and the ethylene randomcopolymers of poly-1-butene, and 90%-0% by weight of the polymercomposition of a polymer selected from the group consisting of ethylenerandom copolymers of polypropylene, butene random copolymers ofpolypropylene and ethylene-butene random terpolymers of polypropylene.

DESCRIPTION OF THE PREFERRED EMBODIMENTS OF THE INVENTION

[0015] A cold seal release film in accordance with this inventionincludes a nucleating agent, preferably sodium benzoate, added to apolymer composition comprising 10%-100% by weight of the polymercomposition of a polymer selected from the group consisting ofpoly-1-butene homopolymers, poly-1-butene/ethylene random copolymershaving over 50% by weight poly-1-butene and blends of the poly-1-butenehomopolymers and ethylene random copolymers of poly-1-butene, and 90%-0%by weight of the polymer composition of a polymer selected from thegroup consisting of ethylene random copolymers of polypropylene, butenerandom copolymers of polypropylene and ethylene-butene randomterpolymers of polypropylene. These films have been determined to haveexcellent cold seal release from a variety of adhesives, includingacrylic/rubber latex adhesives and synthetic adhesives, as well asexcellent scuff resistant properties. Most preferably, the cold sealrelease from the synthetic adhesives is less than 50 grams/inch.

[0016] In accordance with this invention the poly-1-butene/ethylenerandom copolymers include over 50% by weight of the 1-butene monomer.Most preferably the poly-1-butene/ethylene random copolymer includesless than 10% by weight of ethylene and more preferably between 0.1% and6% by weight of ethylene. Representative poly-1-butene/ethylene randomcopolymers usable in this invention are Basell DP8220 and Basell PB8340,which are medium ethylene content and low ethylene content poly-1-butenerandom copolymers, respectively. In particular, Basell DP8220 includesapproximately 2.5% by weight ethylene and Basell PB 8340 includesapproximately 0.2% by weight ethylene. It is believed, based uponexperience, that the poly-1-butene homopolymer will perform in thisinvention the same as the poly-1-butene random copolymer including 0.2%by weight ethylene. Both of the above-identified poly-1-butene/ethylenerandom copolymers are manufactured by Basell Polyolefins of WilmingtonDel. 19808.

[0017] As noted above, the poly-1-butene homopolymers andpoly-1-butene/ethylene random copolymers can be employed alone, can beblended together, or each can be employed in combination with othercopolymers and terpolymers, including ethylene/propylene randomcopolymers, propylene/butene random copolymers andpropylene/ethylene/butene random terpolymers. The preferredethylene/propylene random copolymers are predominately propylenemonomers, by weight, including up to about 10% by weight ethylene. Morepreferably the propylene/ethylene random copolymers include less than 6%ethylene, by weight. A representative propylene/ethylene randomcopolymer usable in this invention is Fina 8573, which includesapproximately 5% by weight ethylene monomer in it. Fina 8573 ismanufactured by ATO FINA Petrochemicals, LaPorte Tex., 77571.

[0018] As noted above, another copolymer that can be employed incombination with either poly-1-butene homopolymer or apoly-1-butene/ethylene copolymer is a propylene/butene random copolymer,wherein the propylene is the major component, by weight, of thecopolymer. Most preferably the propylene/butene random copolymerincludes less than 35% by weight butene and more preferably 3% to 20% byweight butene. A representative propylene/butene random copolymer usablein this invention is Union Carbide DS4DO5, manufactured by DOW Plasticsin Houston Tex. 77042. This propylene/butene random copolymer includesapproximately 14% by weight butene in it.

[0019] A representative propylene/ethylene/butene random terpolymerusable in this invention is Sumitomo SP88. This terpolymer includesapproximately 85% by weight polypropylene, based on the weight of thepolymer composition of the terpolymer, approximately 12% by weightethylene based on the weight of the polymer composition of theterpolymer and approximately 3% by weight butene based on the weight ofthe polymer composition of the terpolymer. This terpolymer ismanufactured by Phillips Sumika, Houston Tex. 77058

[0020] The preferred nucleating agent employed in this invention issodium benzoate; preferably in the range of 0.02-0.1% of the overallpolymer blend. It is considered to be within the scope of this inventionto employ other nucleating agents. Individuals skilled in the art cantest various nucleating agents to determine whether they provideenhanced scuff resistance without adversely affecting the cold sealrelease properties from the adhesive.

[0021] In the context of this invention a nucleating agent is one thathas the ability to raise the crystallization temperature of a polyolefinas measured by Differential Scanning Calorimetry (DSC). The quantity ofthe nucleating agent required to raise the crystallization temperaturewill vary depending upon the specific nucleating agent that is employed.

[0022] In accordance with this invention the cold seal release film canbe a monolayer web or film, or an outer layer of a multilayer,coextruded film. In common packaging applications the monolayer ormultilayer, coextruded release films are adhered to yet another film,which also can be a monolayer or multilayer film having additionaldesired properties for specific packaging applications and alsoreceiving a cold seal adhesive on the outer exposed surface thereof. Forexample, the other film can be a single layer or multilayer white opaquefilm or a metallized film layer. In these latter applications thesurface of the monolayer or multilayer, coextruded release film oppositethe release surface, which is the surface to be laminated to the otherfilm, is treated, preferably oxidatively treated by corona or flametreatment, to make it receptive to laminating adhesives and, optionally,to printing inks. Thus, this latter surface optionally can be reverseprinted so as to be viewable from the outer side of the release layerand also is capable of receiving a laminating adhesive thereon to bondthe monolayer or multilayer release film to the other film, as describedabove.

[0023] In other applications the monolayer or multilayer, coextrudedrelease film may be used by itself, without being laminated to any otherfilms. In this mode of operation, the surface of the film opposite thesurface that is intended to function as the cold seal release surfacepreferably is treated, preferably oxidatively treated by corona or flametreatment, to make this opposite surface receptive to cold sealadhesives and, optionally, to printing inks.

[0024] In accordance with the most preferred embodiments of thisinvention, the cold seal release film is an outer layer of a composite,multilayer, coextruded film including a central core, an outer releaselayer including a nucleating agent combined with the earlier-describedpolymer compositions and a skin layer on the side opposite the releaselayer. As explained above, the outer surface of this latter outer skinlayer is treated, preferably oxidatively treated by corona or flametreatment, to make the surface receptive to printing inks and/orlaminating adhesives and/or cold seal adhesives, depending upon whetherthis outer surface is intended to be reverse printed, laminated to anadditional film that will include the cold seal adhesive on the outerexposed surface thereof, or intended to directly receive the cold sealadhesive.

[0025] Preferably the skin layer opposite the release layer in themultilayer, coextruded release film is a propylene/ethylene copolymerincluding less than 10% ethylene, by weight, and more preferably lessthan 7% ethylene by weight. A preferred propylene/ethylene copolymeremployed as the skin layer is Fina 8573, which, as noted earlier in thisapplication, includes approximately 5% by weight ethylene monomer in it.

[0026] In a preferred embodiment of this invention the core layer is apolyolefin material, or a blend of polyolefin materials, and mostpreferably is a polypropylene homopolymer. A representativepolypropylene homopolymer employed as the core layer is Fina 3371.

[0027] A preferred multilayer construction of this invention is athree-layer structure including a release film or layer having athickness in the range of 0.25 to 20 microns; preferably 1 micron, a20-micron-thick core layer and 1-micron-thick skin layer.

[0028] It should be understood that the films of this invention mayinclude a variety of additives that are well known to those skilled inthe art. For example, antistatic additives, slip additives (e.g., highmolecular weight fatty acid amides), organic or inorganic antiblockingadditives (e.g., organic polymers such as polyamides, polyesters,polycarbonates and inorganic materials such as silicon dioxide andsilicates) and antioxidants may be added to one or more layers of thefilms of this invention, if desired.

[0029] Table 1, infra, more specifically describes the construction andcomposition of seven sample multilayer, coextruded constructions;samples 2, 5 and 6 being within the scope of the present invention. Theremaining samples were prepared for comparison purposes.

[0030] Table 2, infra, shows the cold seal release properties and thescuff resistence properties of the seven samples identified in Table 1.It should be noted that in order to achieve the maximum benefits of thisinvention, care should be taken to introduce the nucleating agentuniformly throughout the composition. This can best be achieved bygrinding the sodium benzoate or other nucleating agent to a very fineparticle size, on the order of less than 10 microns. Most preferably thenucleating agent employed in this invention is sodium benzoate having99.5%, by weight, of a particle size of 10 microns or less. TABLE 1Sample Descriptions Release skin Material Material compositionDescription Core Composition Description Skin composition −1 Fina 3371homopolymer polypropylene Fina 3371 homopolymer Fina 8573 polypropylene−2 Basell DP8220 medium ethylene content (2.5%) Fina 3371 homopolymerFina 8573 0.05% sodium benzoate poly-1-butene polypropylene −3 BasellDP8220 medium ethylene content (2.5%) Fina 3371 homopolymer Fina 8573 0.0% sodium benzoate poly-1-butene polypropylene −4 Fina 3571homopolymer polypropylene Fina 3371 homopolymer Fina 8573  0.3% TospearlT-120 crosslinked polydialkyl siloxane 0.05% behenamide fatty acid amidepolypropylene (antiblock agent) (slip agent) −5   40% Basell DP8220medium ethylene content poly-1-butene Fina 3371 homopolymer Fina 8573  60% Fina 8573 ethylene propylene random copolymer 0.05% behenamidefatty acid amide polypropylene  0.1% sodium benzoate  0.3% TospearlT-120 crosslinked polydialkyl siloxane −6   40% Basell PB8340 lowethylene (0.2 %) poly-1-butene Fina 3371 homopolymer Fina 8573   60%Union Carbide DS4DO butene propylene random copolymer 0.05% behenamidefatty acid amide polypropylene  0.1% sodium benzoate  0.3% TospearlT-120 crosslinked polydialkyl siloxane −7   40% Basell DP8220 mediumethylene content poly-1-butene Fina 3371 homopolymer Fina 8573   60%Fina 8573 ethylene propylene random copolymer 0.05% behenamide fattyacid amide polypropylene  0.0% sodium benzoate  0.3% Tospearl T-1 20crosslinked polydialkyl siloxane

[0031] TABLE 2 Cold Seal Adhesive Release Force g/in 16 hr @ 22 C, 100psi Sutherland Rub test Release Skin Polymer ATOfindley ATOfindleyTechnical Coatings Technical Coatings Initial Final or Polymer Blend1380 7114 339D 357D % Haze % Haze −1 Homopolymer PP 18 30 90 144 1.41.5* −2 Basell DP8220 + 10 20 35 54 5 6.3* Sodium Benzoate −3 BasellDP8220 + 17 nm nm nm 4.5 22.7* NO Sodium Benzoate −4 Homopolymer PP 4135 73 121 1.5 1.5** −5 40% DP8220 + 60% 42 30 35 64 1.2 1.2** Fina8573 + Sodium Benzoate −6 40% PB8340 + 60% 38 24 36 63 1 1.2** DS4D05 +Sodium Benzoate −7 40% DP8220 + 60% 35 28 38 60 1.2 2.5** Fina 8573 + NOSodium Benzoate

[0032] Film Testing Procedure to Determine the Cold Seal AdhesiveRelease Force

[0033] A coating of the specified adhesive was applied via a Mayer rodonto the designated film substrate to provide an adhesive strengthgreater than 300 g/in. The coating was dried and then the release filmwas placed against the adhesive. Next, the two films were placed into ablocking jig where a contact force of 100 psi was applied. The sampleswere held in the jig for 16 hours at 22° C. The force to separate therelease film from the adhesive was then measured.

[0034] Film Testing Procedure to Determine Scuff Resistance

[0035] Two sheets of film with the release surfaces facing each otherwere mounted on the base and sled of a Sutherland rub tester. The twosurfaces were then rubbed for the number of cycles specified in Table 2.The percent of transmission haze was measured before and after theSutherland rub test, and the difference is the measure of scuffresistance, i.e., the lower the difference the better the scuffresistance.

[0036] The invention is illustrated in more detail with reference to thefollowing Examples, but it should be understood that the presentinvention is not deemed to be limited thereto.

EXAMPLES

[0037] The test results reported in Table 2 demonstrate that theaddition of a nucleating agent, e.g., sodium benzoate, improves thescuff resistance of the films of this invention, without adverselyaffecting cold seal release from both acrylic/rubber latex adhesives andsynthetic adhesives.

[0038] Samples 1 through 3 were produced via a laboratory co-extrusiondie and then drawn 7× by 7× at a temperature of 140° C. to effectbiaxial orientation. After orientation, the films were heat set 10%.Simultaneously biaxial orientation was carried out on a T. M. Longstretcher (T. M. Long Corporation, Somerville, N.J.).

[0039] Samples 4 through 7 were produced by a co-extrusion process on apilot tenter machine, with the cross-direction orientation step takingplace after machine direction orientation, and with the machinedirection/cross-machine direction draw ratios being 5× by 10×

[0040] As noted above, samples 1-3 were prepared on the laboratoryextruder and T. M. Long stretcher. Samples 1 and 2 were evaluated withone acrylic/rubber latex (ATOfindley 1380) and three synthetic adhesives(ATOfindley 7114, Technical Coatings 339D and Technical Coatings 357D).The comparative sample 3 was evaluated only with the acrylic/rubberlatex.

[0041] The ATO findley adhesives are manufactured by ATO Findley,Wauwatosa Wis. 53226. The ATOfindley 1380 adhesive is an acrylic/rubberlatex adhesive. The ATOfindley 7114 adhesive is a synthetic adhesive.

[0042] The Technical Coatings adhesives are manufactured by Rohm & Haas,Woodstock Ill. 60098. Both the Technical Coatings 339D adhesive and theTechnical Coatings 357D adhesive are synthetic adhesives.

[0043] The test results of sample 2 versus sample 1 demonstrate that thepoly-1-butene/ethylene copolymer surface has release properties superiorto homopolymer polypropylene for synthetic adhesives, but has aninferior scuff resistance. However, a comparison of sample 2 tocomparative sample 3 demonstrates that the scuff resistance of thenucleated polymer (sample 2) is substantially superior to thenon-nucleated polymer (comparative sample 3).

[0044] Samples 4 through 7 were prepared in a separate set ofexperiments on a pilot tenter machine, as described earlier herein. Inthis sample set a polypropylene control, designated sample 4 wascompared to nucleated blends of poly-1-butene/ethylene copolymers withan ethylene random polypropylene copolymer designated sample 5, and witha butene random polypropylene copolymer designated sample 6. Comparativesample 7 is the same polymer blend as sample 5 but with no sodiumbenzoate nucleator added. Note that due to differences in the manner ofproducing samples 1 and 4 the absolute values of the adhesive releaseforce are different. However, they still are of the same order ofmagnitude; demonstrating poor release from synthetic adhesives.

[0045] A comparison of sample 4 with samples 5 and 6 again demonstratesthat blends containing poly-1-butene/ethylene copolymers yield superiorcold seal release force for synthetic adhesives. Comparison of sample 5with comparative sample 7 again demonstrates that the addition of sodiumbenzoate nucleator improves the scuff resistance of the film (note thatsamples 1 through 3 were run for 50 cycles on the Sutherland rub testerand samples 4 through 7 were run for only 10 cycles).

[0046] While the invention has been described in detail with referenceto specific examples thereof, it will be apparent to one skilled in theart that various changes and modifications can be made therein withoutdeparting from the spirit and scope thereof.

1. A cold seal release film in the form of a monolayer film or an outerlayer of a multilayer film, said cold seal release film comprising anucleating agent and 10%-100% by weight of the polymer composition of apolymer selected from the group consisting of poly-1-butenehomopolymers, poly-1-butene/ethylene random copolymers having over 50%by weight poly-1-butene therein and blends of the poly-1-butenehomopolymers and the poly-1-butene/ethylene random copolymers, and90%-0% by weight of the polymer composition of a polymer selected fromthe group consisting of ethylene random copolymers of polypropylene,butene random copolymers of polypropylene and ethylene-butene randomterpolymers of polypropylene, said polypropylene being over 50% byweight of the polymer composition in said ethylene random copolymers ofpolypropylene and butene random copolymers of polypropylene and beingthe major component by weight of the ethylene-butene random terpolymersof polypropylene.
 2. The cold seal release film of claim 1, wherein 100%of said polymer composition is selected from the group consisting ofpoly-1-butene homopolymers, poly-1-butene/ethylene random copolymershaving over 50% by weight poly-1-butene therein and blends of thepoly-1-butene homopolymers and the poly-1-butene/ethylene randomcopolymers.
 3. The cold seal release film of claim 1, wherein 100% ofsaid polymer composition is a poly-1-butene/ethylene random copolymerhaving over 50% by weight poly-1-butene therein.
 4. The cold sealrelease film of claim 1, wherein said nucleating agent is 0.02% to 0.1%of sodium benzoate by weight based on the weight of the polymercomposition.
 5. The cold seal release film of claim 2, wherein saidnucleating agent is 0.02% to 0.1% of sodium benzoate by weight based onthe weight of the polymer composition.
 6. The cold seal release film ofclaim 3, wherein said nucleating agent is 0.02% to 0.1% of sodiumbenzoate by weight based on the weight of the polymer composition. 7.The cold seal release film of claim 1 being an outer layer of amultilayer film.
 8. The cold seal release film of claim 2 being an outerlayer of a multilayer film.
 9. The cold seal release film of claim 3being an outer layer of a multilayer film.
 10. The cold seal releasefilm of claim 4 being an outer layer of a multilayer film.
 11. The coldseal release film of claim 5 being an outer layer of a multilayer film.12. The cold seal release film of claim 6 being an outer layer of amultilayer film.
 13. The cold seal release film of claim 1, wherein whensaid film is a monolayer film one surface is a release surface and anopposed surface is treated to receive either a cold seal adhesive or alaminating adhesive thereon, and when said film is the outer layer of amultilayer film an outer surface of said multilayer film opposed to theouter surface of the layer constituting the cold seal release film istreated to receive either a cold seal adhesive or a laminating adhesivethereon.